The present invention generally relates to a method and apparatus for performing in vivo analyses of the time-varying absolute radioactivity of selected human organs following intravenous injection or oral administration of a radiopharmaceutical. More particularly, the present invention concerns a nuclear medicine technique and instrument which may be employed to quantitate the absolute activities of a physiologically important radioactive isotope within an organ over a predetermined test period. The time activity relationship can be used to analyze physiological factors--that is, the functions and activity of an organ, including the physical and chemical processes involved--such as to give the regional blood flow to the organ and the metabolic function of the organ as represented by the radioactively labelled drug by the organ. The device also can function to determine the morphological factors--that is, the form and structure--of an organ, such as depth within the patient. This invention is applicable, for example, to the diagnosis of diseases of the thyroid and the kidneys and to the localization of tumors using labelled antibodies.
The prior art is best described with respect to the particular case of devices for diagnostic techniques used on the thyroid gland. The principal function of the thyroid is to regulate the body's general metabolism. For this purpose, it requires iodine (from salt in the diet). In the absence of iodine accumulation, this regulatory function cannot be performed. The assessment of this gland's morphology and physiology by nuclear medicine procedures is one of the oldest clinical procedures. It is practiced in most hospitals and other orgainizations providing medical care which are licensed to handle radioisotopes.
In contrast with the apparatus described herein, prior art instruments currently use imaging and scanning techniques. Tomographic imaging scanners are usually classified into two general categories: planar tomographs, where the image corresponds to the distribution of radioactivity in longitudinal planes parallel to the head-to-toe axis of the patient, and transverse tomographs, yielding an analogous image in cross-sectional planes perpendicular to all planes containing the head-to-toe axis direction. Thus, for example, well-known devices such as the positron camera of Brownell & Burnham and the multiplane scanner of Anger are plane tomographs. Kuhl & Edwards' scanner and the newer ECAT (Emission Computed Axial Tomography) and PET (Position Emission Tomography) axial scanners are transverse tomographs devices. The instrument of Brownell & Burnham, the ECAT and the PET utilize the properties of positron-emission radionuclides which yield gamma radiation from annihilation of the positron and the electron essentially to the site of the radionuclide. The radiation consists of highly energetic gamma photons (511 KeV) ejected 180 degrees apart, thus requiring opposing nuclear counters for their detection.
With the increased knowledge of the biologic effects of ionizing radiations, and the parallel increased applications of tracer techniques, there has taken place a growing concern regarding post-treatment effects of exposure to ionizing radiations and the possible parallel deleterious aftereffects of repeated exposure to low doses of radiation. However, there currently exists no clinically documented evidence or established association between the use of radiopharmaceuticals in the diagnoses of thyroid carcinoma. Nonetheless, the radiation exposure to the organ is cumulative in nature and the physicochemical and medical communities are endeavoring to reduce the radiation dose to the thyroid gland while at the same time preserving diagnostic information value of nuclear medicine techniques. The evolution of radiopharmaceutical developments in preparing short-lived radioisotopes, and biological material labelled with these isotopes, offers new alternatives to traditional approaches. However, when coupled with new concepts in nuclear instrumentation technology, further beneficial advantages could be gained in the areas of radiation dose reduction and enhanced diagnostic accuracy.
The continuing primary use of the iodine isotope I-131 represents comparatively one of the larger increments of radiation dose delivered by nuclear medicine applications. Its diagnostic value is also impaired by the inaccuracy of conventional measurement techniques because of background radiation from surrounding tissue and blood flow and the required correction for attenuation due to the presence of intervening tissue. Thus in current practice, critical corrections for extra-thyroidal neck activity and for radiation attenuation by the intervening tissues between the thyroid and the neck surface are required prior to determining the percent of radio-iodine uptake by the thyroid.
Accordingly, it is an object of the present invention to provide an analytical method and instrument arrangement that enables the organ depth in the body, the absolute radioactivity of this organ, and the radioactivity of the surrounding tissues, to be measured concurrently over the duration of the test.
It is another object of the present invention to provide an analytical method and instrument arrangement that enables the physiologic functions of uptake, retention and excretion of radioactive labelled substrates by the organ, and their relative equilibrium regime, to be assessed concurrently.
It is a further object of the present invention to provide an analytical method and apparatus for obtaining information about the fractional cardiac output to the organ.
Another object of the invention is to reduce the dosages of radiopharmaceuticals required in nuclear medicine techniques.